The present invention relates to communication devices and protocols such as those used in wireless security systems having multiple alarm sensors in communication with one or more control systems; and in particular to such security systems where the transmitter is designed to transmit messages at different power levels.
Wireless transmissions in the United States are regulated by the Federal government. Agencies outside of the United States regulate wireless transmissions, as well. For example, wireless transmissions in Europe are regulated by such agencies as European Telecom Standards Institute (ETSI) and other national agencies. In the United States, the Federal Communications Commission ("FCC") issues federal regulations affecting wireless data transmissions. These regulations are found in the Code of Federal Regulations, Title 47, Part 15 (47 CFR .sctn.15). The term "message", as used herein, generally refers to the information content of a signal, whereas the term "signal" generally refers to the actual transmission and its electromagnetic properties. However, these terms may be used interchangeably in order to promote clarity within a particular context.
The FCC allocates frequency bands for certain types of wireless transmissions. These frequency bands are either "licensed" or "license exempt". If a wireless transmitter is designed to transmit within a frequency for which a license is required (a "licensed frequency"), the operator of the wireless equipment must pay the government a license fee for each radio installed. If a wireless transmitter is designed to transmit within a frequency for which no license is required (a "license exempt frequency"), no license fee is required. For wireless transmission applications having a large number of transmitters, or where the overall wireless information system is inexpensive, the license fee is prohibitively expensive for a commercially competitive product.
The FCC also designates wireless transmissions as either a primary or a secondary user of a frequency. A primary user of a frequency is one that the FCC protects from interference by other transmissions. One example of a primary user is an airport instrument landing system. Such users are protected from interference by not allowing other transmissions on the delegated frequency band because these transmissions guide airplanes to safe landings. Secondary users of frequencies do not enjoy this protection from interference. Secondary frequencies include, for example, frequencies for garage door openers and remote controls. These systems do not involve the same safety considerations that necessitate FCC protection.
The FCC imposes several constraints on license exempt, secondary transmissions. Federal regulations require that the transmissions maximum field strength be calculated as the average field strength over a complete pulse train, provided that the pulse train does not exceed 100 ms. If the pulse train exceeds 100 ms, the field strength is determined as the average absolute voltage over 100 ms. If the emissions are pulsed, rather than constant, the emissions may be transmitted in less than 100 ms, rather than spread out over the entire 100 ms. Because the field strength of pulsed emissions are measured as an average over a 100 ms period, the field strength transmitted during this time may be greater than that which may be transmitted if sent by a constant signal over the entire 100 ms period. For example, if a pulsed signal train is "on the air" for only 50 ms of the 100 ms period, it may be transmitted with double the maximum field strength of a substantially similar constant signal. The FCC limits the transmission of the total allowed power to no less than 10% of the allotted time (i.e., no less than 10 ms). That is, the peak power cannot exceed one hundred times the average allowed power over the entire 100 ms period. These parameters create an incentive to reduce the transmission "on time" to as small as possible (down to 10 ms) to allow for greater transmission power than if the signal was "on" for the entire 100 ms period.
For this reason, certain transmission types are preferred over others for license exempt, secondary transmission applications. For example, frequency modulation (FM) signals are not preferred because they are constantly "on". On-off-keyed amplitude modulation (OOK-AM) is preferred because the signal is off except when data is being transmitted (i.e, not unlike Morse code), and it is inexpensive and simple to implement. Additional encoding schemes which are commonly used include pulse position modulation (PPM) (where the position of a pulse in a self-clocked signal conveys data) and biphase Manchester (where the transition of a signal from one value to another within a bit interval conveys data and the clock).
Constant carrier signals (i.e., constantly "on") are not preferred because they cannot take as much advantage of peak to average power ratio calculation when compared with, for instance, a constant carrier signal such as OOK-AM. For example, a Manchester encoded OOK-AM signal containing 10 bits of information will be on no more than 50% of the on-air time. Thus, the transmission can occupy 20 ms and will have an average "on time" of 10% of the FCC average period. On the other hand, a constant carrier FM signal can occupy only 10 ms periods in order to satisfy the FCC averaging limitation. Therefore, to transmit 10 bits of information, the amplitude modulation can be sent at 10 bits/20 ms=500 Hz data rate, whereas the constant carrier signal must be sent at 10 bits/10 ms=1 KHz in order to take advantage of the full 10% averaging permitted by the FCC. Because the FM data rate is higher, the receiver bandwidths must be wider thus compromising the receiver sensitivity and selectivity.
The PPM and biphase Manchester techniques are often preferred because they efficiently use the power/time (duty cycle) constraints and because they are self-clocking (i.e., the techniques transmit a synchronization pulse or edge from which the position of the data pulse is compared). Self-clocking signals are often preferred because the data timing in the inexpensive transmitters is often poor and, therefore, self-clocking signals are necessary for the receiver or decoder to operate satisfactorily.
Another technique, the non-return-to-zero (NRZ) technique, transmits digital information, and is only "on" (i.e., it only transmits) when the digital signal is a logical "1. This technique is quite efficient and can operate at less than half the data rate of the PPM or biphase Manchester techniques for equal information transfer. This is because in the biphase Manchester technique each bit-cell is divided by 2 to provide the transitions. PPM requires two or more bit cells to convey information depending on the number of possible pulse positions. The NRZ information is conveyed in a single bit-cell, thus, the bit rate equals the baud rate because each bit is equal to the clock rate. However, the NRZ technique duty cycle is entirely dependent on the data content. This means that the more "1's" contained in the digital data signal, the more "on-air" time needed to transmit the signal. Thus, it is at least equally likely that there will be more than 50% "1's". An NRZ signal may not be able to take advantage of the peak-to-average-power ratio because it will be "on air" for more than the minimum regulatory averaging period, reducing the power which may be used to transmit the signal. Also NRZ is not self clocking. For these reasons, NRZ has not previously been preferred for use in prior license exempt, secondary wireless transmission systems.
Governmental and commercial constraints placed on license exempt bands impose conflicting requirements for wireless security systems. The relatively low power level permitted requires (1) as short a transmission on time as possible, and (2) a highly sensitive receiver to reliably detect transmissions at a reasonable distance. The license exempt bands are secondary frequency bands, which have a greater likelihood of interference from other applications in the same band (i.e., the band is not protected from interference). Thus, the receiver must also be highly selective to avoid unwanted interference
Therefore, it would be advantageous if a communications system suitable for use with wireless security systems could provide means for transmitting both data information and control information while at the same time reducing message length and on-air time of the transmitted message. This would be desirable to reduce the probability of message clash (i.e., probability of interference between two or more messages at any given time) and compliance with the high peak to average power ratio as limited in accordance with FCC regulations.
Most radio frequency (RF) wireless security systems available today, such as those manufactured by Alarm Manufacturing Device Co. (ADEMCO) 165 Eileen Way, Syosset, N.Y. 11791, employ a multiplicity of transmitters in communication with a central receiver control unit. The information transmitted typically describes the state of various transducers or sensors associated with each transmitter, such as smoke, motion, breaking glass, shock and vibration detectors; door, window and floor mat switches; etc. These transmitters are designed to be inexpensive to manufacture and generally are capable of transmission only rather than reception only or transmission and reception, which would add significant cost to the design.
The Federal Communications Commission (FCC) regulates operation of low power radio devices under Part 15.231 of the Code of Federal Regulations, which deals with the operation of devices used to control security applications. The regulations specify a maximum permissible power level for control applications as well as a reduced power level including additional duty cycle restrictions for data applications. The difference between the maximum permissible transmission power levels for control and data applications is 8 dB. The additional 8 dB permissible for control application transmissions becomes significant in wireless security systems, which use low power levels for transmission. Data applications are also limited by FCC regulations to no more than one second in duration with a silent period between data transmissions of at least thirty times the duration of the data transmission.
A recent innovation in security applications is the use of two-way wireless consoles or display units. These consoles are portable devices that control and interrogate the security system for status. For example, the 5827BD and 5804BD are two such consoles manufactured by ADEMCO. These wireless consoles are designed to be either portable or permanently installed in a readily accessible location, which reduces the high costs associated with wiring a conventional wired console. One limitation of these devices is that they are intended to operate as control devices as defined in accordance with FCC regulations. As such these devices indicate status by activating a light emitting diode (LED) display, which is simply either on or off.
In contrast, conventional wired consoles are much more sophisticated and often display complex messages descriptive of the system status in a textual format (e.g., "FAULT MASTER BEDROOM WINDOW"). Complex messages such as these are regarded as data messages under FCC regulations. According to FCC regulation, such data messages are not to be transmitted at the same power (i.e., normal power) at which control messages are transmitted. Instead, these data messages can only be transmitted at a reduced power level, which is 8 dB below that of control message transmissions. This reduction in transmission power levels for data messages causes a reduction in both the range and reliability of the security system. Addressing and identification information such as which zone an alarm originated in is permissible within control messages, however, any enhancement of that information required by the security system is considered to be data and requires a reduction in transmitted power.
Therefore, it would be advantageous if a communications system suitable for use with wireless security systems could provide means for transmitting both data information and control information at their respective maximum power levels permitted under FCC regulations, thus maximizing the signal-to-noise ratio (SNR) for all message transmissions and reducing errors in reception.
The reduced power level required for the transmission of data information increases the probability of errors in reception due to the decrease in the SNR as compared with transmissions of control information. During the installation of wireless security systems, when the wireless console can be placed relatively near or at a reduced distance from the control system (e.g., less than six feet), the distance over which data information is transmitted is not limited and thus errors in reception are substantially non-existent. However, during normal operation the user is permitted to use the wireless console at relatively greater distances or at a normal distance (e.g., up to 500 feet) from the control system which may induce errors in the reception of data information at reduced power levels required for transmission of data information.
Therefore, it would be advantageous if a method and apparatus suitable for use with wireless security systems could alleviate the need for transmissions of data information during normal operation of the wireless security system when transmission distances are substantially greater than that experienced during installation. In addition, reducing the quantity of transmissions of data information is advantageous since on-air time is thereby reduced, which results in a reduction in the potential for clash between message transmissions.